This cylindrical cover is to fit over a lingam (or might have been worshipped in its own right in lieu of a lingam). It is of thickly-walled brass with a reinforced rim and a rounded top.
Many covers have faces (in which case they are called a mukhalinga) but this cover is noteworthy for its complete plainness and abstraction.
The lingam is a stylised representation of a phallus and is a tantric representation of Shiva. Usually linga are carved from stone.
Lingam covers are used in puja or prayer rituals and are a visible form of the mantra namah Shivaya through which devotees invoke the blessings of Shiva.
The cover is in excellent condition.
References
Aryan, S., Unknown Masterpieces of Indian Folk and Tribal Art, KC Aryan’s Home of Folk Art, 2005.
Aryan, S., B.N. Aryan & G. Rossi, Mukhalinga, Ethnoarte, 2011.
Rawson, P., Tantra, Arts Council of Great Britain, 1971.